1. Field of the Invention
The present invention relates to wide converter lenses attached to an object side of imaging lenses (main lens systems) used in, for example, digital still cameras, video cameras, or broadcasting cameras, and particularly, to a wide converter lens that shortens the focal length of the entire system relative to the original focal length of the main lens system.
2. Description of the Related Art
As a general technique for shifting the focal length of a main lens system (imaging lenses) towards the wide angle side, or in other words, shortening the focal length, a front-type wide converter lens is known. A wide converter lens of this type is an afocal lens attached to an object side of a main lens system. This technique is advantageous in that the F-number of the main lens system does not change even when the focal length is shifted.
U.S. Pat. No. 2,854,891 discloses an example of a front-type wide converter lens having a three lens structure. Specifically, in this example, the wide converter lens has a first lens element of negative refractive power, a second lens element of negative refractive power, and a third lens element of positive refractive power that are bonded to each other in that order from the object side.
In recent years, there has been disclosed a wide converter lens of a four lens structure for achieving better optical performance, such as an example described in Japanese Patent Laid-Open No. 2000-241700. A wide converter lens of such a four lens structure includes a first lens element of negative refractive power, a second lens element of positive refractive power, a third lens element of negative refractive power, and a fourth lens element of positive refractive power.
Typically, a wide converter lens is constituted by a small number of lens elements to achieve compactness. When such a typical wide converter lens is attached to an object side of a main lens system, the curvature of field, the distortion aberration, and the chromatic aberration of magnification change significantly due to optical characteristics in which off-axis rays transmit through peripheral areas of the lens. On the other hand, when a wide converter lens is attached to a zoom lens, the distortion aberration and the curvature of field often increase at the wide-angle end.
If an object-side lens surface is given a strong positive refractive power to reduce the distortion aberration and the curvature of field at the wide-angle end, the spherical aberration will increase in the negative direction at the telephoto end, thus making it difficult to attain high quality images over the entire zooming range.
The wide converter lens disclosed in U.S. Pat. No. 2,854,891 is detachably attached to a zoom lens and has a three lens structure with a lens element of negative refractive power, a lens element of negative refractive power, and a lens element of positive refractive power arranged in that order from the object side. However, this wide converter lens is problematic in that a large distortion aberration occurs at the wide-angle end and that the chromatic aberration of magnification tends to become large over the entire zooming range.
In addition, when the afocal magnification is reduced, the power of the lens increases entirely. This unfavorably tends to cause an image plane variation to become greater when zooming.
On the other hand, when the wide converter lens of the four lens structure disclosed in Japanese Patent Laid-Open No. 2000-241700 is attached to a zoom lens, improvements in meridional image plane and chromatic aberration of magnification are achieved at the wide-angle end.
However, because the second lens element and the third lens element are separated from each other by a large distance to reduce slanting of a meridional image plane, the effective front-lens diameter is increased, causing an increase in size of the entire lens system.
Consequently, with regard to a wide converter lens attached to a zoom lens, it is desired that an increase in the effective front-lens diameter be prevented, that the entire lens system be reduced in size, and that a high image quality be attained.